Bi-directional transceiver with time synchronization

ABSTRACT

An optoelectronic module may include an optical receiver optically coupled with an optical fiber. The optical receiver may be configured to receive time synchronization signals from the optical fiber. The time synchronization signals may be frequency modulated, wavelength modulated, or amplitude modulated and may be received along with received data signals. A time synchronization signal detection module may be communicatively coupled to the optical receiver. The time synchronization signal detection module may be configured to receive the time synchronization signals that are transmitted through the optical fiber and detect frequency modulations, wavelength modulations, or amplitude modulations to recover the time synchronization signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/189,741, filed on Oct. 9, 2020, which is a continuation of U.S.application Ser. No. 15/691,425, filed on Aug. 30, 2017, and claims thebenefit of and priority to U.S. Provisional Application No. 62/381,546,filed Aug. 30, 2016, both applications are incorporated herein byreference in their entirety.

BACKGROUND

The present disclosure generally relates to time-synchronization fornetworks. Time-synchronization aims to coordinate otherwise independentclocks in different network components.

Many services running on various networks require accurate timesynchronization for correct operation. For example, if switches do notoperate with the same clock rates, then slips will occur and degradeperformance. Many networks rely on the use of highly accurate primaryreference clocks which are distributed network-wide usingsynchronization links and synchronization supply units. Accurate timesynchronization, or phase synchronization, is often needed to supportrequirements for the air interface of some mobile systems. Accurate timesynchronization between different base stations may be important for thenetwork to operate properly. For example, accurate time synchronizationmay facilitate handovers, when a device such as a cell phone istransfers from one base station to another base station, or variousapplications, including location based services, carrier aggregation,coordinated multipoint transmission, and relaying functions. Accuratetime synchronization may also facilitate accurately locating mobiledevices, such as cell phones.

The claimed subject matter is not limited to embodiments that solve anydisadvantages or that operate only in environments such as thosedescribed above. This background is only provided to illustrate examplesof where the present disclosure may be utilized.

SUMMARY

The present disclosure generally relates to time-synchronization fornetworks. In some configurations, time synchronization signals may becombined with the primary and/or data signals travelling through abidirectional optical cable.

In some aspects, a system may include an optical fiber and anoptoelectronic module. The optoelectronic module may include an opticaltransmitter and a controller. The optical transmitter may be opticallycoupled with the optical fiber. The controller may be communicativelycoupled to the optical transmitter. The controller may be configured tooperate the optical transmitter to transmit data signals through theoptical fiber. The optoelectronic module may be configured to transmittime synchronization signals through the optical fiber along with thedata signals.

In some embodiments, an optical multiplexer or demultiplexer may beoptically coupled between the optoelectronic module and the opticalfiber.

In some aspects, time synchronization signals may be amplitudemodulated, frequency modulated, or wavelength modulated to betransmitted through the optical fiber along with the data signals.

The optical fiber may be a bidirectional optical fiber. The system maybe a bidirectional dense wavelength division multiplexing system or abidirectional colorless system, and the system may be configured totransmit data signals and time synchronization signals in a firstdirection and an opposite second direction through the optical fiber.

The controller may frequency modulate, wavelength modulate, or amplitudemodulate the time synchronization signals to be transmitted over theoptical fiber along with the data signals.

The optoelectronic module may include a time synchronization signaldetection module configured to receive received time synchronizationsignals from the optical fiber. The received time synchronizationsignals may be frequency modulated, wavelength modulated, or amplitudemodulated and are received along with received data signals.

The controller may include a power and extinction ratio (ER) controlmodule, the power and ER control module may be configured to modulatethe amplitude of the signals emitted by the optical transmitter tocombine data signals with time synchronization signals. The controllermay include an input stage configured to receive one or more timesynchronization inputs. The controller may include a driver thatreceives data input signals and drives the optical transmitter.

The optoelectronic module may include an optical receiver opticallycoupled to receive optical signals from the optical fiber. Theoptoelectronic module may include a time synchronization signaldetection module configured to detect time synchronization signalsreceived through the optical fiber and output time synchronizationoutput signals.

The optoelectronic module may include a time synchronization signaldetection module that includes an amplifier, a low pass filter coupledto the amplifier; and a limiting amplifier coupled to the low passfilter.

The optoelectronic module may include a power controller that mayinclude a power and extinction ratio (ER) control module, and a driverthat receives data input signals. The optoelectronic module may includea wavelength controller that may include an input stage configured toreceive one or more time synchronization inputs, and a temperature andwavelength control module configured to change the frequency orwavelength of signals emitted by the optical transmitter to transmittime synchronization signals over the optical fiber along with the datasignals.

The optical transmitter may be a tunable laser and the optoelectronicmodule may include a tunable filter optically coupled to an opticalfiber.

The optoelectronic module may include a time synchronization signaldetection module that may include a limiting amplifier, amark-space-ratio detector coupled to the limiting amplifier, and afilter controller coupled to the mark-space-ratio detector. The filtercontroller may be configured to control the tunable filter. Theoptoelectronic module may include a monitor photodiode optically coupledto the tunable filter, and the monitor photodiode may be electricallycoupled to the limiting amplifier.

The optoelectronic module may include a splitter configured to direct aportion of optical signals to the monitor photodiode via the tunablefilter.

The optoelectronic module may include a second optical transmitteroptically coupled with the optical fiber, and a second controllercommunicatively coupled to the optical transmitter. The controller maybe configured to operate the second optical transmitter to transmit thetime synchronization signals through the optical fiber along with thedata signals.

The optoelectronic module may include an optical receiver opticallycoupled with the optical fiber. The optical receiver may be configuredto receive received time synchronization signals from the optical fiber.The optoelectronic module may include a time synchronization signaldetection module communicatively coupled to the optical receiver. Insome aspects, the transmitted time synchronization signals and thereceived time synchronization signals may include different wavelengths.

The optoelectronic module may include a splitter optically coupledbetween the optical transmitter and the optical fiber, and the splittermay also be optically coupled between the optical receiver and theoptical fiber. The splitter may be configured to direct the transmittedtime synchronization signals from the optical transmitter to the opticalfiber; and direct the received time synchronization signals from theoptical fiber to the optical receiver. In some aspects, the transmittedtime synchronization signals and the received time synchronizationsignals may include the same wavelength.

In some configurations, the optical transmitter may be a directlymodulated optical transmitter, a tunable laser with an externalmodulator, a tunable laser with a Mach-Zehnder modulator, and/or atunable laser with an electro-absorption modulator.

This Summary introduces a selection of concepts in a simplified formthat are further described below in the Detailed Description. ThisSummary is not intended to identify key features or essentialcharacteristics of the claimed subject matter, nor is it intended to beused as an aid in determining the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic view of an example of a wireless network withGPS-based time synchronization.

FIG. 2 is a schematic view of a portion of a wireless network.

FIG. 3 is a schematic view of another example of a wireless network.

FIG. 4A is a schematic view of an example of bidirectional colorlesssystem.

FIG. 4B is a schematic view of an example of a bidirectional densewavelength division multiplexing (“DWDM”) system.

FIGS. 5A-5B are schematic views of unidirectional, two-fiber systems.

FIGS. 6A-6B are schematic views of bi-directional, single fiber systems.

FIG. 7 is a schematic view of an example of a timing circuit.

FIG. 8 is a schematic view of an example dual fiber unidirectionalsystem.

FIG. 9 illustrates time synchronization input and output of the systemof FIG. 8.

FIG. 10 is a schematic view of an example of a transceiver.

FIG. 11 is a schematic view of the transceiver of FIG. 10.

FIG. 12 is a schematic view of an example of a bidirectional opticalsubassembly.

FIG. 13 is a schematic view of a portion of the transceiver of FIGS. 10and 11.

FIG. 14 is a schematic view of an example of a transceiver.

FIG. 15 is a schematic view of the transceiver of FIG. 14.

FIG. 16 is a schematic view of an example of a bidirectional opticalsubassembly.

FIG. 17 is a schematic view of a portion of the transceiver of FIGS. 14and 15.

FIG. 18 is a schematic view of another example of a transceiver.

FIG. 19 is a schematic view of another example of a transceiver.

DETAILED DESCRIPTION

Reference will be made to the drawings and specific language will beused to describe various aspects of the disclosure. Using the drawingsand description in this manner should not be construed as limiting itsscope. Additional aspects may be apparent in light of the disclosure,including the claims, or may be learned by practice.

The present disclosure generally relates to time-synchronization fornetworks. Examples of time-synchronization methods and systems aredescribed in “Time and phase synchronization aspects of packet networks”Revised Recommendation ITU-T G.8271/Y.1366, July, 2016, INTERNATIONALTELECOMMUNICATION UNION (available athttps://www.itu.int/rec/T-REC-G.8271-201607-P/en), which is incorporatedby reference in its entirety.

Many services running on various networks require accurate timesynchronization for correct operation. For example, if switches do notoperate with the same clock rates, then slips will occur and degradeperformance. Many networks rely on the use of highly accurate primaryreference clocks which are distributed network-wide usingsynchronization links and synchronization supply units. Accurate timesynchronization, or phase synchronization, is often needed to supportrequirements for the air interface of some mobile systems, as in thecase of time division duplex (TDD) systems (for instance, LTE TDD) orwhen supporting multimedia broadcast/multicast service (MBMS). Accuratetime synchronization between different base stations, e.g., timesynchronization values <1±11 μs, may be important for the network tooperate properly. For example, accurate time synchronization mayfacilitate handovers, when a device such as a cell phone is transfersfrom one base station to another base station, or various applications,including location based services and some LTE-A features. LTE-Afeatures that may benefit from accurate time synchronization may includecarrier aggregation, coordinated multipoint transmission (also known asnetwork MIMO), and relaying functions. Accurate time synchronization mayalso facilitate accurately locating mobile devices, such as cell phones.

Table 1 below lists time and phase requirement classes, as defined inITU-T G.8271 (time and phase synchronization aspects of packetnetworks). The location based services and some LTE-A features mayrequire even higher time synchronization accuracy, at nanosecond levels,which may make it more challenging for system implementation.

Time error with Level of respect to common Typical applications accuracyreference (for information) 1 500 ms Billing, alarms 2 100 μs IP Delaymonitoring 3 5 μs LTE TDD (large cell) 4 1.5 μs UTRA-TDD, LTE-TDD (smallcell) WiMAX-TDD (some configurations) 5 1 μs WiMAX-TDD (someconfigurations) 6 x ns Various applications, including Location basedservices and some LTE-A features

For many conventional wireless networks, time synchronization is basedon information from a global positioning system (“GPS”). FIG. 1 isschematic view of an example of a wireless network 100 with GPS-basedtime synchronization. As illustrated, the network 100 includes basestations 102 communicatively coupled to backhaul access networks 104 viatransport equipment 106. In some configuration, the base stations 102and the backhaul access networks 104 may exchange Fast Ethernet (FE) orGigabit Ethernet (GE) signals. The backhaul access networks 104 arecommunicatively coupled to a backhaul metro network 108. The backhaulmetro network 108 may be communicatively coupled to an Evolved PacketCore (EPC) or Wireless Core Network 110. The networks 104, 108, 110 andthe base stations 102 may represent different layers of the network 100.

Timing information may be provided by standby clock source 112 and/orstandby clock source 114. In particular, the clock sources 112, 114 maygenerate clock signals that are provided to various components in thenetwork 100. In the illustrated configuration, the clock signals aredistributed using GPS antennas 116, 118. Each of the base stations 102of the network 100 may include a GPS antenna to obtain timinginformation via the GPS antenna 118. Such configurations may beimplemented for time-division duplex (“TDD”) systems (such as thosedeployed in China) or code division multiple access (“CDMA”) systems(such as those deployed in the United States or other countries). ForGPS-based time synchronization, the signals used for timesynchronization may include a pulse per second (“1PPS”) signal and timeof date (“TOD”) information.

FIG. 2 is a schematic view of a portion of a wireless network 200. Asillustrated for example in FIG. 2, in some conventional networksemploying GPS-based time synchronization, a GPS receiver 202 may becommunicatively coupled to a GPS antenna 205 to remotely receive signalsfrom a GPS antenna 204. A regenerator 206 may be coupled between the GPSantenna 205 and the GPS receiver 202 to amplify the GPS signals. Ananti-lighting module 208 may be included to protect components fromlightning or other electrical disturbances. The GPS receiver 202 mayreceive the GPS signals and transmit 1PPS and TOD signals to a basestation 210, for example, via a cable. In some configurations, the GPSreceiver 202 may be connected to the base station 210 or imbedded in thebase station 210. In some circumstances, the 1PPS and TOD signals mayhave an accuracy of around 200 nanoseconds.

In some circumstances, the configuration illustrated in FIG. 2 may beimplemented in the network 100 of FIG. 1. In particular, theconfiguration illustrated in FIG. 2 may be implemented to provide timesynchronization signals for the base stations 102 of FIG. 1.

Networks that implement GPS-based time synchronization may have goodaccuracy, for example, around 30 nanoseconds, but deployment costs forsuch configurations may be relatively high. Specifically, use of manyGPS antennas/receivers may make implementing GPS-based timesynchronization relatively expensive.

FIG. 3 is a schematic view of another example of a wireless network 300.The network 300 may include any suitable aspects of the network 100, andsimilar or same features are indicated with similar numbering.

As illustrated for example in FIG. 3, some networks, such as the network300, may include optical transceivers 302 to transmit data through atleast part of the network using optical signals. In particular, thenetwork 300 implements duplex transceivers 302 coupled by optical fibers304, 306. In some configurations, the network may be a unidirectionaloptical system. Such systems are configured to transmit optical signalsin one direction over a single, first optical cable and transmit signalsin an opposite second direction over a second cable different from thefirst optical cable. Such systems may be considered “unidirectional”because each optical cable is used to transmit optical signals in onlyone direction. Unidirectional optical systems may implement the duplextransceivers 302 coupled by the two optical fibers 304, 306, one of theoptical fibers 304 for transmitting data in a first direction, and asecond of the optical fibers 306 for transmitting data in an oppositesecond direction through the network. In some circumstances, the firstand second directions may be referred to as east/west directions.

Some networks, for example, networks that include duplex transceivers,may implement the Precision Time Protocol (PTP) synchronization protocolas defined in the IEEE 1588-2002 standard or PTP Version 2 protocoldefined in the IEEE 1588-2008 standard (“IEEE 1588 v2”). In PTP Version2, time synchronization may be performed at least partially by sending asynchronization signals between a master unit and a slave unit. Forexample, the master unit may send a signal to a slave unit, and a slaveunit sends a response to the master unit. However, in suchconfigurations there may be latency resulting from the time it takes forthe signal to travel through a transmission medium, such as an opticalcable. This latency must be accounted for in the time synchronizationscheme for accurate time synchronization.

To account for this latency, the master unit may receive the responseand calculate the time difference or delay between the time the signalwas sent and the time the response was received. The time difference maybe divided in half to estimate the latency is a first direction (masterto slave) as well as a second direction (slave to master).

This delay may represent a latency across the communications medium(i.e., the optical fiber) so that the time synchronization system maycompensate for this delay. However, such an estimation is only accurateif the latency in the first direction and the second direction areapproximately equal. If the latency in the first direction is differentfrom the latency in the second direction, this may cause issues for timesynchronization because the time synchronization system may not properlyaccount for the difference in latency. Differences in latency may becaused by differences in length between the first communications medium(e.g., an east optical fiber) and the second communications medium(e.g., a west optical fiber), or if the length of the first or secondcommunications medium changes for some reason. Furthermore, differencesin latency may be caused by transceiver wavelength change in the firstor second directions. In addition, it may not be practicable or possibleto implement PTP Version 2 for unidirectional backhaul systems, becauseit may not be possible to send signals between a master unit and a slaveunit. In some circumstances, such errors will result in incorrectlatencies to be calculated in at least one direction.

The latency in either the first direction or the second direction maychange when the length of one of the optical fibers changes. In somecircumstances, the latency change may be represented by the followingformula:

Latency change=D (ps/nm/km)*delta_wavelength (nm)*L (km)

In this formula, D represents the chromatic dispersion coefficient orconstant, delta_wavelength represents the wavelength variation of theoptical signal, and L represents the length of the transmission mediumor optical fiber.

Generally, networks may include a main fiber cable, which may runthrough different locations within a city or between different cities.The networks may further include relatively short optical cables, whichmay be described as jumper cables, to connect equipment to the network.For unidirectional systems, if one of the fiber cables is severed, thefiber cable may be fused back together to restore the connection.However, it may be difficult to control and/or account for the latencychange resulting from the change in fiber length in thesevered/reconnected fiber, because the length would likely change whenthe optical cable is fused and the connection is restored. Also, the twooptical cables for the two directions would likely no longer be theexact same length. So if the fiber is ever severed and reconnected in aunidirectional dense wavelength division multiplexing (“DWDM”) system,the change in fiber lengths may have an impact on the timesynchronization accuracy.

The latency difference in both directions may be calibrated and/oraccounted for during system setup, but latency may further change aftersetup, for example, during maintenance, which will degrade timesynchronization accuracy. This would even apply to relatively shortoptical cables, for example, optical cables located inside of abuilding, which may be referred to as “jumper cables.” If the fiberjumper cable were changed, for example, from a 1 meter cable to a 2meter cable then the time synchronization would also be changed. In suchcircumstances, an operator of network may have the difficult task ofmonitoring the length of all of the optical fibers or other transmissionmedia, and compensating for their length, particularly when the lengthschange. This may make maintenance of the network very difficult becausean operator would need to monitor the length of all of the portions ofall optical fibers or other transmission media.

In some embodiments, a bidirectional system may be employed to mitigatethe issue of monitoring the lengths of the optical fibers or othertransmission media. For example, the bidirectional system may employ onefiber for both the west and the east link, so the length in bothdirections will always be the same. FIGS. 4A and 4B are examples ofbidirectional systems.

FIG. 4A is a schematic view of an example of bidirectional colorlesssystem 400. As illustrated, the system 400 may include bidirectionaltransceivers 404 and 406 configured to transmit optical signals over anoptical fiber 402. In the illustrated configuration, the transceivers404, 406 are bidirectional, meaning they send and receive opticalsignals in two directions over the optical fiber 402 (e.g., east andwest directions).

Each of the transceivers 404, 406 may be configured to convertelectrical signals to optical signals to be transmitted through theoptical fiber 402, and may be configured to receive optical signals andconvert the optical signals to electrical signals. The transceiver 404includes transmitter 410, a receiver 412, and a filter 414. Thetransmitter 410 generates optical signals that are transmitted throughthe filter 414 and into the optical fiber 402 and travel through theoptical fiber 402 to the transceiver 406. The transceiver 404 mayreceive optical signals (for example, from the transceiver 406), thereceived optical signals may be directed to the receiver 412 by thefilter 414, and the receiver 412 may convert the optical signals toelectrical signals.

The transceiver 406 includes transmitter 420, a receiver 422, and afilter 424. The transmitter 420 generates optical signals that aretransmitted through the filter 424 and into the optical fiber 402 andtravel through the optical fiber 402 to the transceiver 404. Thetransceiver 406 may receive optical signals (for example, from thetransceiver 404), the received optical signals may be directed to thereceiver 422 by the filter 424, and the receiver 422 may convert theoptical signals to electrical signals.

As illustrated, the bidirectional transceivers of FIG. 4A communicateoptical signals between one another over the optical fiber 402. Each ofthe bidirectional transceivers 404, 406 of FIG. 4A include an opticaltransmitter 410, 420 to transmit optical signals over the optical fiber402 and an optical receiver 412, 422 (“RX”) to receive optical signalsfrom the optical fiber 402. Each of the bidirectional transceivers alsoinclude a filter 414, 424, a splitter or other suitable opticalcomponent that directs optical signals between the transmitter,receiver, and the optical fiber. The filters 414, 424 permit opticalsignals from the transmitter to travel through the filter to the opticalfiber 402. The filters 414, 424 reflect signals received from theoptical fiber 402 and direct the signals to the receivers 412, 422.Although one example of an optical filter is shown, other opticalconfigurations may be implemented to direct light signals as may beappropriate.

FIG. 4B is a schematic view of an example of a bidirectional densewavelength division multiplexing (“DWDM”) system 450. Rather thanincluding one transceiver on each side of an optical cable, such as thesystem of FIG. 4A, the system 450 includes multiple transceivers 454 onone side of an optical fiber 452, and multiple transceivers 456 on theother side of the optical fiber 452. Each of the transceivers 454include a transmitter 560, a receiver 462, and a filter 464, and each ofthe transceivers 456 include a transmitter 570, a receiver 472, and afilter 474 configured to exchange optical signals via the optical fiber452. In the illustrated configuration, the system 450 includes 40channels, with 40 corresponding transceivers 454 on one side of theoptical fiber 452 and 40 transceivers 456 on the other side of theoptical fiber 452 (only two of which are illustrated in detail, forclarity). However, the system 450 may include any suitable number ofchannels are corresponding transceivers. In some configurations, each ofthe channels of the system 450 have different wavelengths. In suchconfigurations, each of the transceivers 454, 456 may be configured totransmit and receive a different wavelength of optical signals, and, inparticular, each of the transceivers 454, 456 may be configured totransmit/receive a specific wavelength or range of wavelengths.

The system 450 also includes an optical multiplexer/demultiplexer(mux/demux) 480 on one side of the optical fiber 452 and a mux/demux 482on the other side of the optical fiber 452. The mux/demux 480 receivesthe different optical signals (e.g., different channels) from thetransmitters 460 of the transceivers 454 and combines the opticalsignals to be transmitted through the optical fiber 452. The mux/demux482 receives the combined optical signals from the transceivers 454 andseparates the optical signals to be received by the correspondingreceivers 472 of the transceivers 456. Similarly, The mux/demux 482receives the different optical signals (e.g., different channels) fromthe transmitters 470 of the transceivers 454 and combines the opticalsignals to be transmitted through the optical fiber 452. The mux/demux480 receives the combined optical signals from the transceivers 456 andseparates the optical signals to be received by the correspondingreceivers 462 of the transceivers 464.

As illustrated in FIG. 4B, the system 450 communicates optical signalsfrom the different optical transceivers 454, 456 over the optical fiber452. The system 450 includes the optical mux/demux 480, 482 that directsoptical signals between the different transceivers 454, 456. AlthoughFIG. 4B illustrates four of the transceivers 454, 456 in detail, thesystem 450 may include any suitable number of transceivers, with eachpair of transceivers corresponding to one channel of optical signalsthat may travel through the optical fiber. The illustrated system 450may include any suitable number of channels, but in the illustratedembodiment it includes 40 channels. Each of the channels may beassociated with a different wavelength of light.

The systems illustrated in FIGS. 4A-4B may be implemented in variousnetworks to transmit optical signals. For example, the systemsillustrated in FIGS. 4A-4B may be implemented in the network 100 of FIG.1 or the network 300 of FIG. 3. The systems illustrated in FIGS. 4A-4Bare bidirectional systems, meaning they are configured to transmitsignals in a first direction and an opposite second direction over thesame optical cable. This is in contrast to unidirectional systems.

Since the systems in 4A-4B use a single optical fiber for bothdirections of optical signal travel, the length of the optical fiber isthe same for both directions of communication. Accordingly, the latencyacross the communications medium may be approximately the same in bothdirections. Furthermore, the time difference for signals to travel fromthe master to the slave, and back from the slave to the master may bedivided in half to accurately estimate the latency through thecommunication medium (in this case, the optical fiber).

FIGS. 4A-4B are merely schematic representations, and the transceiversand/or optical systems may normally include other optical and electricalcomponents as may be appropriate.

FIGS. 5A-5B are schematic views of a unidirectional, two-fiber systemswhere IEEE1588V2 time synchronization is implemented in a MAC PHY (MediaAccess Control Physical) layer, which is at the physical layer ofEthernet link. In particular, FIG. 5A is a schematic view of a system500. FIG. 5A represents the timing accuracy of a dual fiberunidirectional system 500, such as the system 300 of FIG. 3. As shown,timing signals may be transmitted from a master clock 502 via a GPS 504to a base stations 506. As the timing signals travel, they accumulatelatency delay, and timing accuracy decreases as the timing signalstravel through different nodes and mediums.

The timing accuracy of the system 500 may be represented by the formula:

Accuracy=|ΔT1+ΔT2+ΔT3|

In this formula, ΔT1 represents the time synchronization accuracy ofmaster clock, ΔT2 represents the time synchronization accuracy oftransmission system, and ΔT3 represents the time synchronizationaccuracy between baseband control unit(s) (BBU) and remote radio unit(s)(RRU). In some configurations, the total timing accuracy of this systemmay be less than 1.5 microseconds, or around 1.5 microseconds. In somecircumstances, ΔT2 per node may be less than 30 microseconds. Since theillustrated system is unidirectional, it may require compensation forfiber length change. ΔT2 per node may depend on the fiber lengthdifference between the east optical fiber and the west optical fiber.

FIG. 5B is a schematic view of an example dual fiber unidirectionalsystem 520. As illustrated, the system 520 includes duplex transceivers522, 524 without time synchronization capabilities optically coupledwith two optical fibers, one optical fiber 526 for transmitting opticalsignals in a first direction and a second optical fiber 528 fortransmitting optical signals in an opposite second direction. The system520 also includes framers, 532, 534, with the framer 532 correspondingto the transceiver 522 and the framer 534 corresponding to thetransceiver 534. The framers 532, 534 may distinguish timesynchronization signals from other data signals, thereby permitting thetime synchronization signals to be extracted for decoding orretransmission.

The framers 532, 534 may also verify the information in the timesynchronization signal. The framers 532, 534 may also transmit timesynchronization signals. Once the time synchronization signals areextracted they may be transmitted to a corresponding IEEE1588 module536, 538. As illustrated, one of the IEEE1588 modules 536 associatedwith the transceiver 522 may be a master IEEE1588 module and the otherIEEE1588 module 538 associated with the transceiver 524 may be a slaveIEEE1588 module. The IEEE1588 modules 536, 538 may use the received timesynchronization signals for time synchronization. In suchconfigurations, the master IEEE1588 module 536 may provide timesynchronization signals to the framer 532, which may combine the timesynchronization signals with a main signal, which is then transmitted tothe transceiver 522, and sent over the optical fiber 526 and received atthe transceiver 524. The transceiver 524 sends the main signal with thetime synchronization signals to the framer 534, which separates the mainsignal from the time synchronization signals. The framer 534 sends thetime synchronization signals to the slave IEEE1588 module 538.

FIGS. 6A-6B are schematic views of a bi-directional, single fibersystems where IEEE1588V2 time synchronization is implemented in a MACPHY (Media Access Control Physical) layer. In particular, FIG. 6A is aschematic view of a system 600. FIG. 6A represents the timing accuracyof a bi-directional, single fiber system, such as the systems 400, 450of FIGS. 4A-4B. As shown, timing signals may be transmitted from amaster clock 502 via a GPS 504 to a base stations 506. As the timingsignals travel, they accumulate latency delay, and timing accuracydecreases as the timing signals travel through different nodes andmediums.

The timing accuracy of the system of FIG. 6A may be represented by theformula:

Accuracy=|ΔT1+ΔT2+ΔT3|

In this formula, ΔT1 represents the time synchronization accuracy ofmaster clock, ΔT2 represents the time synchronization accuracy oftransmission system, and ΔT3 represents the time synchronizationaccuracy between baseband control unit(s) (BBU) and remote radio unit(s)(RRU). In some configurations, the timing accuracy of this system may beless than 1.5 microseconds, or around 1.5 microseconds. In somecircumstances, ΔT2 per node may be less than 30 microseconds. Since theillustrated system is bidirectional, compensation for fiber lengthchange may not be required. ΔT2 per node may depend on the fiber lengthdifference between the east optical fiber and the west optical fiber.

FIG. 6B is a schematic view of an example single fiber bidirectionalsystem 620. The system 620 of FIG. 6B may be a ground systemimplementing IEEE1588V2 over a single fiber bidirectional system. In theillustrated configuration, IEEE1588V2 may be implemented in a mediaaccess control (MAC) physical layer.

As illustrated, the system 620 includes transceivers 622, 624 withouttime synchronization capabilities optically coupled with a singleoptical fiber 626. The system 620 also includes framers 632, 634corresponding to each transceiver 622, 624. The framers 632, 634 maydistinguish time synchronization signals from other data signals,thereby permitting the time synchronization signals to be extracted fordecoding or retransmission. The framers 632, 634 may also verify theinformation in the time synchronization signal. Once the timesynchronization signals are extracted they may be transmitted to acorresponding IEEE1588 module 636, 638. As illustrated, one of theIEEE1588 modules 636 associated with the transceiver 622 may be a masterIEEE1588 module and the other IEEE1588 module 638 associated with theother transceiver 624 may be a slave IEEE1588 module. The IEEE1588modules 636, 638 may use the received time synchronization signals fortime synchronization.

FIG. 7 illustrates a schematic view of an example of a timing circuit700. In particular, FIG. 7 illustrates how random read/write offirst-in-first-out (FIFO) buffer can degrade time synchronizationaccuracy. As illustrated, a transmitter side 702 of the timing circuit700 may include a signal process unit 710, a transmit first-in-first-out(FIFO) buffer 712, a phase-locked loop (PLL) circuit 714, and a physicalserializer circuit (PHY Serializer) 716. A receiver side 704 of thetiming circuit 700 may include a physical de-serializer circuit (PHYde-serializer) 720, a receiver first-in-first-out (FIFO) buffer 722, asignal process unit 724, and a clock data recovery (CDR) circuit 726. Asillustrated in FIG. 7, Ch1˜chN may represented buses of the main signal.

The signal process unit 710 and/or 724 may be part of a host that anoptical transceiver is plugged into. In some configurations, the signalprocess unit 710 and/or 724 may include, or function as, the framer, asdescribed above. The framer of the signal process unit 710 and/or 724may process the main signal, for example, by inserting IEEE1588package(s) into main data signal stream and/or de-inserting/recoveringIEEE1588 package(s) from the main data signal stream.

Together, the transmit FIFO 712, the receiver FIFO 722, the PLL 714, CDR726, PHY serializer 716 and PHY de-serializer 720 may provide signalinterworking between very high speed serial links at the physical layer,and lower speed buses that the host is capable of processing.

As an IEEE1588 package is inserted into the physical layer, the transmitFIFO 712 on transmitter side 702 and the receiver FIFO 722 on thereceiver side 704 may cause extra phase variation. If we assume that thesignal rate of Ch1˜ChN is ˜311 MHz, then the phase difference ofdifferent combinations due to FIFO effects may be represented asfollows: phase variation of different states of FIFO may be N*1/311 MHz,if FIFO depth is set as minimum. The maximum latency change may bearound N* the interval of the bus signal, because there may be Npossibilities of FIFO read/write during interworking between theparallel bus and the serial main signal. Accordingly, the latencyvariation due to random read/write at the FIFO may be:

102 ps for 10 Gbps PHY signal

257 ps for 25 Gbps PHY signal

1028 ps for 100 Gbps PHY signal

FIG. 8 illustrates a schematic view of an example dual fiberunidirectional system 800. The system 800 may be a ground systemimplementing IEEE1588V2 over a single fiber bidirectional system. In theillustrated configuration, IEEE1588V2 may be implemented over a specificelectrical path in each of the bidirectional modules. As illustrated,the system 800 includes bidirectional transceivers 822, 824 without timesynchronization capabilities optically coupled with a single opticalfiber 826.

The system 800 also includes framers 832, 834 corresponding to eachtransceiver 822, 824. The framers 832, 834 may receive synchronizationsignals from a specific electrical path, thereby permitting the timesynchronization signals to be extracted for decoding or retransmission.The framers 832, 834 may also verify the information in the timesynchronization signal. The framers 832, 834 may be similar or identicalto the framers 632, 634 of FIG. 6B, except in the configuration of FIG.8, a separate input and output signal is used to ensure more accuratetime synchronization. In particular, the framer 832 provides timesynchronization signals to the transceiver 822, and the main data signalis provided to the transceiver separately. Similarly, the framer 834receives time synchronization signals from the transceiver 824, and themain data signal is separate from the time synchronization signals. Insuch configurations, the synchronization signal bit rate may be muchlower than the main signal, and the random read/write of FIFO relatedtime synchronization errors (see FIG. 7 and associated description) maybe decreased or eliminated. For example, time synchronization errors maybe decreased or eliminated because the bit-rate of main signal is thesame or approximately the same as the bit-rate of time synchronizationpackets. Conversely, the higher the signal bit rate, the larger the timesynchronization change may be due to interworking between the parallelbus and the serial main signal because of the random read/write errorsof FIFO as described above.

Once the time synchronization signals are extracted they may betransmitted to a corresponding IEEE1588 module 836, 838. As illustrated,one of the IEEE1588 modules 836 associated with the transceiver 822 maybe a master IEEE1588 module and the other IEEE1588 module 838 associatedwith the other transceiver 824 may be a slave IEEE1588 module. TheIEEE1588 modules 836, 838 may generate/receive the time synchronizationsignals for time synchronization.

The timing accuracy of the system 80 of FIG. 8 may be represented by theformula:

Accuracy=|ΔT1+ΔT2+ΔT3|

In this formula, ΔT1 represents the time synchronization accuracy ofmaster clock, ΔT2 represents the time synchronization accuracy oftransmission system, and ΔT3 represents the time synchronizationaccuracy between baseband control unit(s) (BBU) and remote radio unit(s)(RRU). In some configurations, the timing accuracy of this system may beless than 1.5 microseconds, or around 1.0 microseconds. In somecircumstances, ΔT2 per node may be less than 30 microseconds. ΔT2 maydepend on the fiber length difference in the east and the westdirections. Since the illustrated system is bidirectional, compensationfor fiber length change may not be required. ΔT2 per node may depend onthe fiber length difference between the east optical fiber and the westoptical fiber. The illustrated configuration may facilitate accuratetime synchronization, as may be required for some networks.

In the illustrated systems, the time synchronization signal may becombined with the main data transmission signal to be transmitted overthe optical fiber 826. For example, in one configuration the timesynchronization signal may be amplitude modulated. In suchconfigurations, the time synchronization signal may be added to the maindata transmission signal by envelope modulation of the optical signalwith a low modulation amplitude, which can be performed by modulatingthe bias current of the transmitter laser, or by modulating the pumpcurrent of a semiconductor optical amplifier (SOA) following thetransmitter laser, or by other means to vary the average power of theoptical signal. In any case, the time synchronization signal can bedetected by monitoring the slow power variation of the optical signal ona (relatively) low-bandwidth photo diode at the Rx direction.

In another example, the time synchronization signal may befrequency/phase modulated. In such configurations, the timesynchronization signal can be added by frequency/phase modulation of theoptical signal with a low modulation amplitude, which can be performedby modulating the DBR current of a tunable DBR laser, or by modulatingthe bias current of the transmitter laser. The time synchronizationsignal can be detected by adding one edge filter of the optical signalwith an extra photo diode at the receive direction.

In some circumstances, the bit width of the time synchronization signalmay be limited if the time synchronization signals are amplitudemodulated. Furthermore, some configurations where amplitude modulatedtime synchronization signals are implemented may introduce cross talk tothe data signal. In such circumstances, frequency/phase modulated timesynchronization signals may be preferable.

In some of the disclosed embodiments, the time synchronization channelis separate from the main signal. Accordingly, the bit-rate of the timesynchronization signal will not change along with the main signal.Therefore, the random read/write related time synchronization errors(see FIG. 7 and associated description) may be decreased or eliminated,and time synchronization may be improved.

FIG. 9 illustrates the time synchronization input and output of thesystem 800 of FIG. 8. Inside each of the transceivers 822, 824 there maybe components that affect time synchronization accuracy such as traces,PCBs, flex circuits, or other components that may impact timesynchronization, and the impact on time synchronization may becompensated by assigning a delay or latency to such components. Forexample, as illustrated, on an input side 900 a PCB/flex 902 mayintroduce a delay of 250 ps*5%, a driver 904 may introduce a delay of 10ps, and/or an optical component 906 may introduce a delay of 10 ps orless. Similarly, on an output side 910 an optical component 912 mayintroduce a delay of 10 ps or less, an amplifier 914 may introduce adelay of 10 ps, and a PCB/flex 916 may introduce a delay of 250 ps*5%.

As illustrated, such components may be compensated for on both the inputside and the output side. The delays introduced by the components may beadded together to obtain a total latency change or delay. In somecircumstances, this may be a round trip peak to peak latency variation.

In addition to delays introduced by the components, an extra delay maybe caused by wavelength variation of optical signal(s). Such variationsmay also be included and/or accounted for in the round trip peak to peaklatency variation.

In the bidirectional fiber cable, the wavelength of signals traveling inthe east and west directions may be different from one another. In somecircumstances, the latency difference between the west and eastdirections can be compensated based on wavelength differences.Specifically, since the nominal wavelength for both directions may beknown, the changes in wavelength, or the wavelength difference, may beused to calculate the latency difference.

In some circumstances, the latency difference may be represented by thefollowing formula:

Latency change=D (ps/nm/km)*delta_wavelength (nm)*L (km)

In this formula, D represents the chromatic dispersion coefficient orconstant, delta_wavelength represents the wavelength variation of theoptical signal, and L represents the length of the transmission mediumor optical fiber. In one example, if the wavelength change(delta_wavelength) is +/−0.08 nm, the chromatic dispersion coefficientmay be 17 ps/nm/km, and the latency change may be 17 ps/nm/km*(0.08*2)nm*L_fiber km. This may represent the latency change with compensationfor fixed wavelength differences for the west and east links.Accordingly, a fiber 920 may introduce a delay based on the aboveformula.

For bidirectional DWDM systems, the latency difference of west and eastdirection can be compensated by the wavelength differences. Forun-cooled laser colorless bidirectional transceiver, latency change dueto the wavelength drift of laser can be compensated by the followingformula:

wavelength drift=laser wavelength slope to bias current*temperaturechange

In this formula, laser wavelength slope to bias current represents howmuch wavelength change occurs when the bias current changes, and thetemperature change represents the temperature change of the laser orlaser chip. In some circumstances, the laser wavelength slope to biascurrent may be around 0.08 nm/C. The temperature of the laser may bedetected by thermal resistor positioned sufficiently close to the laser

The latency change due to the PCB A/flex may be compensated based on thedelay introduced by those components. In some configurations, the lengthof the traces or other electrical lines in the transceiver may beconfigured to be substantially the same length on the transmitter andreceiver sides of the transceiver. Such configurations ensure that thelatency is the same in both directions. In some configurations, thelatency change may be identified and/or compensated for by detectingPCBA temperature and calculating the latency change based on thematerial dielectric constant change.

FIG. 10 is a schematic view of an example of a transceiver 1000 that maybe used to implement the time synchronization schemes described above.For example, the transceiver 1000 may be used to implement amplitudemodulation of the time synchronization signals. The transceiver 1000 mayinclude a controller 1002 and a laser and modulator 1004. The controller1002 may be configured to control the laser and the modulator 1004. Thelaser and the modulator 1004 may be coupled to a MUX/DEMUX 1006configured to multiplex and demultiplex optical signals. An opticalfiber 1010 may be coupled to the MUX/DEMUX 1006. The transceiver 1000may also include a photodiode 1008 which may be a PIN or an avalanchephotodiode. The photodiode 1008 may be coupled to a transimpedanceamplifier (TIA) 1012, a time synchronization signal recovery module1014, and a receiver power monitor 1016. The transceiver 1000 my includeconnections for transmit electrical signals TD+TD− and receiveelectrical signals RD+RD−.

Although in the illustrated configuration the laser and the modulator1004 are included together, in other configurations the laser and themodulator may be separate components. In addition, the laser may be anysuitable optical transmitter.

As illustrated, in some configurations, the transceiver 1000 may includeadditional connections for time synchronization signals TX_TimeSync+,TX_TimeSync−, RX_TimeSync+, RX_TimeSync−. In the illustratedconfiguration, the transceiver 1000 includes two couplings for timesynchronization signals TX_TimeSync+, TX_TimeSync− on the transmit sideof the transceiver 1000 and two couplings for time synchronizationsignals RX_TimeSync+, RX_TimeSync− on the receiver side of thetransceiver 1000.

The time synchronization signals on the transmit side may be transmittedto the controller 1002 for the laser and modulator 1004, which in turnoperates a laser and/or modulator 1004 to transmit data signals and timesynchronization signals over the bidirectional optical fiber 1010 (viathe MUX/DEMUX 10006 if the system is a DWDM system). The controller 1002may amplitude modulate the time synchronization signals to betransmitted over the optical fiber 1010 along with the data signals.

The time synchronization signals RX_TimeSync+, RX_TimeSync− on thereceiver side may be recovered by the time synchronization signalrecovery module 1014 coupled to the photodiode 1008 or other suitablereceiver (e.g., avalanche photodiode). In some circumstances, the timesynchronization signal recovery module 1014 may include a low passfilter to recover time synchronization signals. The data signals may bereceived at the photodiode 1008 and may be sent to the TIA 1012 forfurther processing/modulation. The transceiver 1000 may also include thepower monitor 1016 to monitor received signals. In some configurations,the transceiver 1000 may be an SFP+ transceiver.

FIG. 11 is a schematic view of the transceiver 1000 of FIG. 10 infurther detail. FIG. 11 illustrates some additional details of thetransceiver 1000, showing, for example, electrical and opticalcomponents that may be included. As illustrated, the controller mayinclude a power and extinction ratio (ER) control module 1100 and adriver 1104 that drives the integrated laser and modulator 1004. Thepower and ER control module 1100 may change the power of the driver 1104to modulate the amplitude of the signals emitted by the laser andcombine the data signals with the time synchronization signals. Thepower and ER control module 1100 may change the extinction ratio (ER) ofthe driver 1104 to modulate the amplitude of the signals emitted by thelaser 1004 and combine the data signals with the time synchronizationsignals. The transceiver 1000 may also include a wavelength control 1102that modulates the wavelength of the laser 1004.

The transceiver 1000 may include optical components such as variouslenses, isolators and/or filters. As illustrated, the optical componentsmay be included as part of the bidirectional MUX/DEMUX 1006. Thetransceiver 1000 may include a filter 1020 that permits optical signalsfrom the integrated laser and modulator 1004 on the transmit side (TX)of the transceiver 1000 to pass through the filter 1020 and betransmitted through the optical fiber. A lens 1022 may be positionedbetween the integrated laser and modulator 1004 and the filter 1020. Alens 1026 may be positioned on the other side of the filter 1020, forexample, between the filter 1020 and an optical fiber. The filter 1020may reflect signals received from the optical fiber and may direct thesignals to the receiver side (RX) of the transceiver 1000. The receivedsignals (RX) may pass through another filter 1028 and/or a lens 1030 andmay pass to the photodiode 1008 or other suitable receiver. The timesynchronization signal detection module 1014 may detect thepower/amplitude/modulation changes caused by the power & ER controlmodule 1100 to recover the time synchronization signals. The datasignals may travel to the TIA 1012 to be output from the transceiver.

FIG. 12 is a schematic view of an example of a bidirectional opticalsubassembly (BOSA) 1200 that may be implemented the time synchronizationschemes described above. For example, the BOSA 1200 may be used toimplement amplitude modulation of the time synchronization signals. Asillustrated, the BOSA 1200 may include a laser 1202 that emits opticalsignals. The optical signals may travel through a lens 1203 and/or anisolator 1204 to a splitter 1206. The splitter 1206 may direct at leasta portion of the optical signals to a monitor photodiode (MPD) 1208, andat least a portion of the optical signals may pass through the splitter1206 to a filter 1210, the filter 1210 may permit the optical signals topass through the filter 1210, through a lens 1211 and into an opticalfiber 1214. The filter 1210 may reflect signals received from theoptical fiber 1214 and may direct the signals through a lens 1213,another filter 1212, to a receiver/photodiode 1216. The receiver 1216may include an avalanche photodiode or other suitable receiver.

FIG. 13 is a schematic view of a portion of the transceiver 1000 ofFIGS. 10 and 11 in further detail. As illustrated, the timesynchronization signal recovery module 1014 may include an amplifier1300, a low pass filter 1302, and a limiting amplifier 1304. Theamplifier 1300 may receive signals from the photodiode 1008 and mayamplify the signals. The low pass filter 1302 may be configured tofilter out certain signals received from the photodiode 1008, forexample, the time synchronization signals. The limiting amplifier 1304may allow signals (such as the time synchronization signals) below aspecified input power or level to pass through it unaffected whileattenuating the peaks of stronger signals that exceed this threshold.

FIG. 14 is a schematic view of an example of a transceiver 1400 that maybe used to implement the time synchronization schemes described above.For example, the transceiver 1400 may be used to implement frequencymodulation of the time synchronization signals. The transceiver 1400 mayinclude components or features discussed above, and componentspreviously described are indicated with the same numbers for brevity. Asillustrated, in some configurations, the transceiver 1400 may includeadditional connections for time synchronization signals, similar to thetransceiver 1000 of FIG. 10.

The transceiver 1400 may include the controller 1002 which operates thelaser and/or modulator 1004 to transmit data signals over thebidirectional optical fiber 1010 (via the MUX/DEMUX 1006 if the systemis a DWDM system).

The time synchronization signals on the transmit side may be transmittedto the wavelength controller 1102 which may be coupled to the laser andmodulator 1004. The wavelength controller 1102 may change thefrequency/wavelength of signals emitted by the laser and modulator 1004.The wavelength controller 1102 may frequency modulate or wavelength thetime synchronization signals to be transmitted over the optical fiber1010 along with the data signals.

On the receiver side, the transceiver 1400 may include a filter 1402that detects changes in frequency/wavelength to filter out the timesynchronization signals to be transmitted to a dedicated monitorphotodiode (MPD) 1404 or other receiver suitable to receive the timesynchronization signals. The monitor photodiode 1404 is coupled to atime synchronization detection module 1406 that may be configured toreceive, amplify, and/or process the time synchronization signalsreceived by the monitor photodiode 1404.

The transceiver 1400 also includes the avalanche photodiode 1008(PIN/APD) or other receiver suitable to receive the data signals fromthe optical fiber 1010. The photodiode 1008 may be coupled to thereceiver power monitor 1016 and/or the TIA 1012 which may receive thedata signals.

FIG. 15 is a schematic view of the transceiver 1400 of FIG. 14 infurther detail. FIG. 15 illustrates some additional detail of thetransceiver 1400, showing, for example, electrical and opticalcomponents that may be included.

In the illustrated embodiment, the integrated laser and modulator 1004may include a tunable laser. The transceiver 1400 may also include atemperature and wavelength control module 1408. The temperature andwavelength control module 1408 may be configure to change thefrequency/wavelength of signals emitted by the tunable laser of theintegrated laser and modulator 1004. The temperature and wavelengthcontrol module 1408 may frequency modulate the time synchronizationsignals to be transmitted over the optical fiber along with the datasignals.

The transceiver 1400 may include optical components such as variouslenses, isolators and/or filters, such as the lenses 1024, 1026, 1030,the filters 1020, 1028, and the isolator 1022. As illustrated, theoptical components may be included as part of the bidirectionalMUX/DEMUX 1006. The transceiver 1400 may also include a splitter 1410.The received signals (RX) may pass through the splitter 1410 thatdirects at least a portion of the received signals to the filter 1028and the lens 1030. At least a portion of the received signals may passthrough the filter 1028 and the lens 1030 to the photodiode 1008 orother suitable receiver. The splitter 1410 may also direct at least aportion of the received signals through a tunable filter 1402, which mayinclude a filter 1412 and/or a lens 1414. The received signals maytravel through the tunable filter 1402 to the monitor photodiode 1404coupled to the time synchronization signal detection module 1406. Insome configurations, the filter 1402 may be tunable because wavelengthand/or frequency of the time synchronization signals may change based ontemperature or other factors. The filter 1402 may be tuned to suitablyrecover all of the time synchronization signals received at thetransceiver. The synchronization signal detection module 1406, alongwith the tunable filter 1402, may recover the frequency/wavelength tunedtime synchronization signals to be output from the transceiver 1400, orfrom another transceiver.

FIG. 16 is a schematic view of an example of a bidirectional opticalsubassembly (BOSA) 1600 that may be implemented the time synchronizationschemes described above. For example, the BOSA 1600 may be used toimplement frequency modulation of the time synchronization signals. Thetransceiver 1600 may include components or features discussed above, andcomponents previously described are indicated with the same numbers forbrevity.

As illustrated, the BOSA 1600 may include a tunable laser 1601 thatemits optical signals. The optical signals may travel through the lens1203 and/or the isolator 1204 to the splitter 1206. The splitter 1206may direct at least a portion of the optical signals to the MPD 1208,and at least a portion of the optical signals may pass through thesplitter 1206 to the filter 1210, the filter 1210 may permit the opticalsignals to pass through the filter 1210, through the lens 1211 and intothe optical fiber 1214. The filter 1210 may reflect signals receivedfrom the optical fiber 1214 and may direct the signals to a splitter1602. The splitter 1602 may direct a portion of the signals through thelens 1213, the filter 1212 to the receiver/photodiode 1216. The receiver1216 may include an avalanche photodiode or other suitable receiver. Thesplitter 1602 may also direct a portion of the signals to a tunablefilter 1604. The signals may pass through the tunable filter 1604 to atime synchronization monitor photodiode 1606, or other suitablereceiver. The time synchronization signals may be received at the timesynchronization monitor photodiode 1606.

FIG. 17 is a schematic view of a portion of the transceiver 1400 ofFIGS. 14 and 15 in further detail. As illustrated, the timesynchronization signal recovery module 1406 may include a limitingamplifier 1702, a mark-space-ratio detector 1704 and a controller 1706.The limiting amplifier 1702 may allow signals (such as the timesynchronization signals) below a specified input power or level to passthrough it unaffected while attenuating the peaks of stronger signalsthat exceed this threshold.

The mark-space-ratio detector 1704 may detect the mark-space-ratio ofreceived signals and transmit this information to the controller 1706.The controller 1706 may be configured to control the tunable filter1402. In some configurations, the controller 1706 may control thetunable filter 1402 to achieve a 50% mark-space-ratio.

FIG. 18 is a schematic view of another example of a transceiver 1800.The transceiver 1800 may include components or features discussed above,and components previously described are indicated with the same numbersfor brevity. In the illustrated configuration, the transceiver 1800 usesdifferent wavelengths to transmit and/or receive time synchronizationsignals and main data signals. Accordingly, the transceiver 1800includes an additional power controller 1802 and an additional laser andmodulator 1804 to transmit time synchronization signals. The powercontroller 1802 receives electrical time synchronization signals andcontrols the laser and modulator 1804 to generate time synchronizationsignals which are directed to the optical fiber 1010 via the MUX/DEMUX1006.

The transceiver 1800 also includes an additional monitor photodiode(MPD) 1808 and a time synchronization signal detection module 1806communicatively coupled to the MPD 1808. The MPD 1808 may receive timesynchronization signals from the optical fiber 1010 via the MUX/DEMUX1006. The time synchronization signal detection module 1806 that may beconfigured to receive, amplify, and/or process the time synchronizationsignals received by the monitor photodiode 1808.

In the configuration illustrated, the transceiver 1800 may use twoseparate wavelengths (or ranges of wavelengths) to transmit and receivetime synchronization signals. For example, the transceiver 1800 maytransmit time synchronization signals with a first wavelength (or rangeof wavelengths) in a first direction (e.g., east) over the optical fiber1010, and may receive time synchronization signals with a secondwavelength (or range of wavelengths) in a second direction (e.g., west)over the optical fiber 1010.

FIG. 19 illustrates an example of a transceiver that may send andreceive time synchronization signals with the same wavelength (or rangesof wavelengths). FIG. 19 is a schematic view of another example of atransceiver 1900. The transceiver 1900 may include components orfeatures discussed above, and components previously described areindicated with the same numbers for brevity.

As illustrated, the transceiver 1900 includes a splitter 1902 positionedbetween the MUX/DEMUX 1006, the laser and modulator 1804 and the MPD1808. The splitter 1902 may separate traffic in two opposite directionsinside a transceiver 1900. In particular, the splitter 1902 may directoptical synchronization signals generated by the laser and modulator1804 into the optical fiber 1010 via the MUX/DEMUX 1006. The splitter1902 may also direct optical synchronization signals from the opticalfiber 1010 to the MPD 1808 via the MUX/DEMUX 1006. In suchconfigurations, the received optical synchronization signals may includethe same wavelength or range of wavelengths as the opticalsynchronization signals generated by the laser and modulator 1804.

In some circumstances, the configuration of the transceiver 1800 of FIG.18, with different wavelengths for different directions, may make iteasier to split the optical synchronization signals from the main datasignals because the signals may be separated by the MUX/DEMUX 1006 basedon wavelength. However, in some circumstances the splitter 1902 may be apower splitter, and may direct 50% of optical signals from the laser andmodulator 1804 to the optical fiber 1010, and/or may direct 50% ofoptical signals from optical fiber 1010 to the MPD 1808. Suchconfigurations may also be relatively simple or inexpensive toimplement.

In some circumstances, the configuration of the transceiver 1800 or thetransceiver 1900 may be implemented in point to point links with SFP+transceivers or other suitable transceivers. In some configurations, thelaser and modulator 1804 used for the time synchronization signals inthe transceivers 1800, 1900 may implement an inexpensive and/orcost-effective laser so as not to significantly increase costs ofimplementing time synchronization through dedicated channels. In suchconfigurations, a separate physical channel may be used to transmit thetime synchronization signals.

In some configurations, the optical transmitters, lasers, and/or themodulators described herein may be directly modulated opticaltransmitters, directed modulation optical transmitters and/or directedmodulator optical transmitters. For example, the laser and the modulator1004 may include a directed modulation optical transmitter or a directedmodulator optical transmitters. In other configurations, the opticaltransmitters, lasers, and/or the modulators described herein may includea tunable laser with an external modulator. For example, the laser andthe modulator 1004 may include a tunable laser with an externalmodulator. In some configurations, the modulators described herein mayinclude a Mach-Zehnder modulator. The Mach-Zehnder modulator may be usedto control or modulate the amplitude of the optical signals, forexample, to transmit time synchronization signals along with main datasignals. In other configurations, the modulators described herein mayinclude an electro-absorption modulator. The electro-absorptionmodulator may be used to modulate the intensity of the optical signals,for example, to transmit time synchronization signals along with maindata signals.

The time synchronization configurations described herein may be usedinstead of GPS-based time synchronization configurations. In theconfigurations described herein, time synchronizations signals aretransmitted separately from the main data signals. However, in some ofthe described configurations the time synchronizations signals aretransmitted via the same transmission medium (e.g., optical fiber) as isused for the main data signals, but on different channels (e.g.,different wavelength channels). In such configurations, changes to themain signal may not affect the time synchronizations signals.Accordingly, time synchronization signal accuracy may be maintainedthroughout the network implementing the concepts described. Furthermore,the described configurations may be implemented in different types ofnetworks that require time synchronization, and the variousconfigurations described may meet module requirements and/or networkrequirements for different systems.

Although the above concepts are described in the context oftransceivers, and the transceivers may include both an opticaltransmitter and an optical receiver, the concepts described herein maybe implemented in any suitable optoelectronic modules. For example, insome configurations optoelectronic modules implementing the conceptsdescribed herein may include one more optical transmitters and nooptical receivers, or one more optical receivers and no opticaltransmitters. Any suitable optoelectronic device may be adapted toimplement the concepts described herein.

The terms and words used in the description and claims are not limitedto the bibliographical meanings, but, are merely used to enable a clearand consistent understanding of the disclosure. It is to be understoodthat the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a component surface” includes reference to one or more ofsuch surfaces.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to thoseskilled in the art, may occur in amounts that do not preclude the effectthe characteristic was intended to provide.

Aspects of the present disclosure may be embodied in other forms withoutdeparting from its spirit or essential characteristics. The describedaspects are to be considered in all respects illustrative and notrestrictive. The claimed subject matter is indicated by the appendedclaims rather than by the foregoing description. All changes which comewithin the meaning and range of equivalency of the claims are to beembraced within their scope.

What is claimed is:
 1. An optoelectronic module comprising: an opticalreceiver optically coupled with an optical fiber, the optical receiverconfigured to receive time synchronization signals from the opticalfiber, wherein the time synchronization signals are frequency modulated,wavelength modulated, or amplitude modulated and are received along withreceived data signals; and a time synchronization signal detectionmodule communicatively coupled to the optical receiver, the timesynchronization signal detection module configured to receive the timesynchronization signals that are transmitted through the optical fiberand detect frequency modulations, wavelength modulations, or amplitudemodulations to recover the time synchronization signals.
 2. Theoptoelectronic module of claim 1, wherein the time synchronizationsignals are amplitude modulated.
 3. The optoelectronic module of claim1, the time synchronization signal detection module comprising: anamplifier; a low pass filter coupled to the amplifier; and a limitingamplifier coupled to the low pass filter.
 4. The optoelectronic moduleof claim 3, wherein the amplifier receives the time synchronizationsignal from the optical receiver and amplifies the signals, the low passfilter filters out the time synchronization signals, and the limitingamplifier allows the time synchronization signals below a specifiedinput level to pass through it and attenuates peaks of the timesynchronization that exceed the specified input level.
 5. Theoptoelectronic module of claim 1, further comprising: a receiver powermonitor coupled to the optical receiver; and a transimpedance amplifiercoupled to the optical receiver, the transimpedance amplifier configuredto process or modulate data signals received by the optical receiver. 6.The optoelectronic module of claim 1, further comprising: an opticaltransmitter optically coupled with the optical fiber; and a controllercommunicatively coupled to the optical transmitter, the controllerconfigured to operate the optical transmitter to transmit data signalsthrough the optical fiber.
 7. A system comprising: the optoelectronicmodule of claim 1; and an optical multiplexer or demultiplexer opticallycoupled between the optoelectronic module and the optical fiber.
 8. Thesystem of claim 7, wherein the optical fiber is a bidirectional opticalfiber.
 9. The system of claim 7, wherein the system is a bidirectionaldense wavelength division multiplexing system or a bidirectionalcolorless system, and the system is configured to transmit data signalsand time synchronization signals in a first direction and an oppositesecond direction through the optical fiber.
 10. An optoelectronic modulecomprising: an optical receiver optically coupled with an optical fiber,the optical receiver configured to receive time synchronization signalsfrom the optical fiber, wherein the time synchronization signals areamplitude modulated and are received along with received data signals;and a time synchronization signal detection module comprising: anamplifier; a low pass filter coupled to the amplifier; and a limitingamplifier coupled to the low pass filter.
 11. The optoelectronic moduleof claim 10, wherein time synchronization signal detection modulereceives the amplitude modulated time synchronization signals that aretransmitted through the optical fiber and detects amplitude modulationsto recover the time synchronization signals.
 12. The optoelectronicmodule of claim 10, wherein the amplifier receives the timesynchronization signal from the optical receiver and amplifies thesignals.
 13. The optoelectronic module of claim 12, wherein the low passfilter filters out the time synchronization signals.
 14. Theoptoelectronic module of claim 13, wherein the limiting amplifier allowsthe time synchronization signals below a specified input level to passthrough it and attenuates peaks of the time synchronization that exceedthe specified input level.
 15. The optoelectronic module of claim 10,further comprising: a receiver power monitor coupled to the opticalreceiver; and a transimpedance amplifier coupled to the opticalreceiver, the transimpedance amplifier configured to process or modulatedata signals received by the optical receiver.
 16. The optoelectronicmodule of claim 10, further comprising: an optical transmitter opticallycoupled with the optical fiber; and a controller communicatively coupledto the optical transmitter, the controller configured to operate theoptical transmitter to transmit data signals through the optical fiber.17. A system comprising: the optoelectronic module of claim 10; and anoptical multiplexer or demultiplexer optically coupled between theoptoelectronic module and the optical fiber.
 18. The system of claim 17,wherein the optical fiber is a bidirectional optical fiber.
 19. Thesystem of claim 17, wherein the system is a bidirectional densewavelength division multiplexing system or a bidirectional colorlesssystem.
 20. The system of claim 17, wherein the system is configured totransmit data signals and time synchronization signals in a firstdirection and an opposite second direction through the optical fiber.